The present invention relates to an LED lamp using light-emitting diodes and a method of manufacturing the same lamps. More specifically, this invention relates to LED lamps to be arranged in a matrix forming an outdoor display panel and a method of manufacturing the LED lamps for the same purpose. The outdoor display panel, which is formed by a matrix of the LED lamps according to the present invention, is controlled by a driving IC in such a manner that LED lamps located at designated positions in the matrix are switched on to emit single or multi-color light to display characters, figures and/or an image. It is usable for a lamp display panel such as for example a road signboard, an outdoor signboard or an advertising signboard.
An LED display panel for displaying characters, figures or images is usually constructed by a matrix of equally spaced LED lamps capable of emitting single or multi-color light. Characters, figures or images are displayed on the display panel by selectively driving LED lamps arranged thereon in accordance with the selected lamp positions and colors.
However, any display panel installed outdoor may suffer the affection of the sunlight that enters the LED lamps and, after reflection and refraction, emerges again from the lamps together with light emitted from the switched-on lamps. As the result of this, light synthesized of light emitted from the switched-on LED lamps and reflected sunlight from the LED lamps affects eyes of each observer. Accordingly, it is needed to provide means for facilitating recognition of characters displayed on the outdoor display panel with due consideration of the locations of the sun and the installation conditions of the panel.
To solve the above problem concerning the locations of sun and installation conditions of the display panel, there has been proposed such a conventional display panel wherein spaces formed between LED lamps arranged in a matrix in the display panel are covered with black resin layers and spaces between respective rows of the lamps arranged in the horizontal direction are partially covered with louvers to reduce the quantity of the sunlight that may fall on the lamps.
Referring to FIGS. 1A, 1B and 1C, there is shown a typical construction of a conventional display panel 100. FIG. 1A is a schematic perspective illustration of the conventional display panel 100 viewed from the side direction and FIG. 1B is a schematic perspective illustration of the same panel viewed from the upper right direction. FIG. 1C is a schematic sectional view of the same display panel 100.
FIG. 2 is a schematic sectional view of an LED lamp usable for the conventional display panel shown in FIGS. 1A, 1B and 1C. A number of the LED lamps are arranged at approximately equal intervals in a matrix forming a display panel.
As shown in FIGS. 1A and 1B, the display panel 100 with LED lamps (i.e., LED display unit) incorporates a matrix of LED lamps 1 approximately equally spaced from each other. The spaces between the LED lamps are covered with black resin films 3 formed thereon to absorb sunlight for reducing reflected sunlight. This also produces the enhanced contrast between the light emitting from switched-on LED lamps and the reflected sunlight.
As shown in FIG. 1C, there are arranged louvers 2 one per space between two rows of LED lamps 1 in the matrix to restrict an incident angle of the sunlight directed to the LED lamps 1. This may also increase the contrast between the light emitted from LED chips in the LED lamps 1 and the reflected sunlight from the LED lamps.
Generally, the sunlight incident to each of the LED lamps 1 includes direct rays and rays scattered and refracted by particles such as dust, water drops and gas particles while it traveled through the atmosphere from the sun. It is recognized that the direct incident light of the sun is more intensively reflected from the LED lamp 1.
In early morning or evening when the sun locates low in the sky and the sunlight travels a longer distance through the atmosphere to the display panel, the sunlight that is visible light having long wavelengths and is not easily subjected to diffusion and refraction by particles such as dust, water and gas may directly enter the LED lamps 1 of the display panel without being shut off by the louvers 2. As the result of this, the ratio of intensity of the reflected sunlight increases, resulting in decreasing the contrast between the light emitted from the switched-on LED lamps and the sunlight reflected from the inside of the switched-off LED lamps.
To improve the contrast, louvers may be elongated to decrease an incident angle of the sunlight to the display panel and thereby shorten the duration of decreasing the contrast of an image displayed on the display panel.
Referring now to FIGS. 3A and 3B, the conventional LED lamp 1 (FIG. 2) with a lead frame for supply electric power to terminals of LED chips in the lamp will be described below in detail. FIG. 3A is a schematic sectional view of the conventional LED lamp using a lead frame and FIG. 3B is a schematic perspective view of a light-reflecting cup 11 used in the LED lamp 1 of FIG. 3A.
As shown in FIG. 3A, the LED lamp 1 uses extending lead frames 15 serving as terminals for supplying electric power to LED chips 14 incorporated in a transparent resin-made package 16 having a cannonball shape suitable for collecting radiant light rays.
In FIGS. 3A and 3B, there is shown the LED lamp 1 of the type that it emits multi-colored light from two LED chips 14 for different colors disposed in the light reflecting cup 11 mounted on the top of a common lead frame 15a forming a common electric connection circuit for the LED chips.
Each LED chip 14 is connected at one electrode terminal (anode or cathode) to one of separate lead frames 15b (for different colors) with a lead wire 17 bonded thereto and connected at the other electrode terminal (cathode or anode) to the common lead frame 15a by using conducting adhesive. The common frame 15a forms for example a ground terminal.
As described above, the light reflecting cup 11 mounted on the top of the common lead frame 15a is intended to effectively collect the light emitted from the LED chips 14 disposed therein in the direction of light path. For this purpose, as shown in FIG. 3B, its side wall surrounding the LED chips 14 is slanted in the light path direction at an angle suitable to collect the light emitted from the LED chips with no interference with the light emitted from the neighboring LED lamps 1. The height of the slanted side wall of the light reflecting cup 11 is also adjusted to ensure a necessary angle of visibility.
As described above, the display panel 100 according to the prior art is provided with louvers 2 and black surface layers 3 on the panel 100 as shown in FIGS. 1A to 1C to reduce the amount of the sunlight reflected from the inside of each LED lamp. In daytime while the sun locates high in the sky, the contrast between the light emitted from the LED chips 14 and the reflected sunlight may be well adjusted. However, in the early morning or evening when the sun locates low in the sky, the sunlight may enters the LED lamps at an incident angle of 15 degrees or less without being shut off by the louvers 2 because the louvers cannot be further adjusted in relation to the angle of visibility. The sunlight having entered the LED lamps 1 on the display panel repeats reflection and refraction and then goes out of the lamps 1 as considerably intensive reflected light together with the light emitted from the LED chips 14 of the switched-on LED lamps.
As a result of this, the relative intensity of the light emitted from the LED chips 14 of switched-on LED lamps 1 to the reflected sunlight from switched-off LED lamps 1 on the display panel 100 decreases. Furthermore, the relative intensity of the light emitted from the LED chips 14 of the switched-on LED lamps 1 to the light not absorbed and reflected from the black surface portions 3 also decreases.
Consequently, the contrast between the light emitted from the LED chips 14 of the switched-on LED lamps 1 and the reflected sunlight from non-light-emitting portions including the switched-off LED lamps 1 on the display panel 100 decreases, making it difficult for observers to see any character, figure or image displayed on the display panel 100 in a specified duration of time under a specified site condition.
In other words, the conventional display panel using LED lamps encountered various problems in its applications regarding a considerable decrease in contrast between the light emitted from the LED chips of the switched-on LED lamps and the sunlight reflected from non-light-emitting portions including the switched-off LED lamps on the display panel. For example, a considerable decrease in contrast of an image may occur on the display panels installed along a street and a slight curved portion of a highway, which may be resulted from, for example, locations of the panel, the observer and the sun and/or the installation angle of the panel. Furthermore, the same contrast problem may occur on the display panels installed on a bay-bridge highway or snowfield road where reflected sunlight is particularly intensive.